1. Field of the Invention
This invention relates to the production of gypsum board. More particularly, the present invention relates to the use of foamed starch pellets in the production of lightweight gypsum board.
2. Description of the Background Art
Gypsum board is one of the most widely used and versatile building materials in the world. The general construction of gypsum board includes a core of calcium sulfate dihydrate that is sandwiched between opposing paper sheets. The core is initially deposited in the form of a slurry; namely, calcium sulfate hemihydrate (CaSO4.½H2O) in water. Once the slurry is deposited, it is rehydrated to form gypsum. The hemihydrate is initially prepared in a mill via the following reaction:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2O(steam)
The dehydrated calcium sulfate is known as calcined gypsum, stucco, or plaster of Paris. Gypsum has a number of desirable physical properties that make it suitable for use as a building material. These properties include fire resistance, compressive strength and neutral pH. Gypsum is also a beneficial building material because it can be formed into various shapes and is inexpensive and plentiful.
It is also known in the art to use additives with gypsum. One such additive is starch. Starch can be added prior to rehydration. Starch functions as a binder within a set gypsum and yields boards with higher compressive and flexural strength. It also strengthens the edges of the resulting board and improves paper bond to the core.
It is further known in the art to form voids with the interior of gypsum board as a means for reducing the board weight. The background art includes several examples of void formation. One technique is described in U.S. Pat. No. 6,706,128 to Sethuraman. Sethuraman '128 discloses a method for adding air bubbles of different relative stabilities, whereby the air bubbles do not rupture before the slurry sets sufficiently to prevent the slurry from filing the void spaces left behind by ruptured bubbles. The result is a gypsum board with reduced weight.
Another example is illustrated in U.S. Pat. No. 1,776,325 to Robinson. Robinson '325 discloses a method of making a cellular wall board by incorporating an aerated starch into a slurry. As a result of the aerated starch, the slurry is given a desired porosity and forms a cellular core.
Finally, U.S. Pat. No. 5,643,510 to Sucech discloses a method for producing foamed gypsum board using a foaming agent blend. The ratio between a first and second foaming agent are controlled to adjust the sizes of foam voids within a gypsum core.
Although each of the above referenced inventions achieves its individual objective, they all suffer from common drawbacks. Namely, the voids are formed via the use of foaming agents require additional chemicals to be added to the gypsum. Void formation added via aeration likewise requires additional machinery. Moreover, in both methods the control of the associated machinery is difficult. In prior methods of void formation it is also difficult to form voids of the correct size.